Milk is generally treated prior to consumption to destroy undesirable and injurious bacteria and spores.
Well known is the process of pasteurization whereby milk may be treated according to either of two known procedures. According to the "flash" process, milk is subjected to relatively high temperatures (e.g. about 70.degree.-75.degree. C.) for a relatively short period of time after which it is refrigerated until time for consumption. Another known pasteurization process is the "holding" process wherein the milk is subjected to a relatively lower temperature and held at that temperature for a longer period of time. Such processes are well known in the art and have been the subject of improvement over the years as evidenced by the processes described in U.S. Pat. Nos. 364,579; 716,819; 1,196,357; 1,714,597; 1,798,413 and 3,054,684. Currently, fluid milk is pasteurized by heat treatments of whole milk at temperatures ranging from 62.degree. to 75.degree. C. for periods of from 30 minutes to as low as 15 seconds. This treatment is of course necessary to destroy disease-producing bacteria and check the activity of the spoilage bacteria found in raw whole milk. However, all bacteria in raw whole milk are not killed by such a treatment, and thus the milk so treated has a limited shelf-life and must be refrigerated at temperatures near 4.degree. C. during all shipping, marketing and storing.
Recently, processes have been developed for the production of ultra-high temperature (UHT) sterilized milk (Aggarwal, M. L., 1974. "Commercial sterilization and aseptic packaging of milk products," J. Milk Food Technol. 37:250; Samuelsson, E. G., and S. Holm. 1966, "Technological principles for ultra-high-heat treatments of milk," XVII Int. Dairy Congr., Munchen, Section B1:57; and Speck, M. L., and F. F. Busta, 1968, "Sterilization and aseptic packaging of milk products--Microbiological trends", J. Dairy Sci. 51:1146).
While UHT processing of milk may have a slightly different meaning in the United States and Canada from that used in Europe, each involves the treatment of milk at temperatures ranging from 120.degree. to about 150.degree. C. for periods up to about 8 seconds which is sufficient to sterilize the milk. One would expect such sterile milk to have an extended shelf-life without requiring refrigeration. However, research has shown that defects in UHT treated milk occasionally develop sooner than expected (See, for example, Biryukova, Z. A., V.I. Seleznes, E. I. Dombrovskaya, and A. I. Makarova. 1974, "Changes in sterilized milk during storage," Food Sci. Technol. Abstr. 6:163; Murthy, L., E. O. Herreid, and R. McL. Whitney, 1958. "Electrophoretic properties of casein from sterilized milk stored at different temperatures", J. Dairy Sci. 41:1324.). Thus, such UHT treated milk has been found to develop a bitter flavor and/or to coagulate (i.e. gelation). The coagulation and bitter flavor are thought to be caused by bacterial protease(s) which survive the ultra-high temperature treatment (See for example Adams, D. M., J. T. Barach, and M. L. Speck. 1975, "Heat resistant proteases produced in milk by psychrotrophic bacteria of dairy origin", J. Dairy Sci. 58:828; Bengtsson, K., L. Gardhage, and B. Isaksson. 1973, "Gelation in UHT treated milk, whey and casein solution; The effect of heat resistant proteases", Milchwissenschaft 28:495; Malik, A. C., and A. M. Swanson. 1974, "Heat-stable proteases from psychrotrophic bacteria in milk," J. Dairy Sci. 57:591. (Abstr.) and Samuelsson, E. G., and S. Holm. 1966, "Technological principles for ultra-high-heat treatments of milk", XVII Int. Dairy Congr., Munchen, Section B 1:57). It has been suggested that the spoilage which results from the development of bitter taste or coagulation could be overcome by increasing the processing heat treatment. However, in light of the high heat resistance which has been reported for these bacterial proteases, a treatment which would destroy the protease would render the product quality unacceptable, and furthermore would be prohibitive from a cost standpoint.
We have discovered a low temperature heat treatment to inactivate heat-resistant bacterial proteases in ultra-high temperature treated milk.
Accordingly, it is the primary object of our invention to provide a process whereby the heat-resistant proteases appendant to ultra-high temperature treated milk may be destroyed.
It is a further object of our invention to provide a means for the production of a marketable fluid milk which is safe from deterioration and spoilage due to heat-resistant bacterial proteases.
A still yet further object of the present invention is to provide a means whereby one may prolong the shelf-life of UHT sterilized milk.
Yet another object of the present invention is to provide a means for the processing of ultra-high temperature treated milk which will remain wholesome without refrigeration and possess a good flavor which to the average consumer cannot be distinguished from freshly pasteurized, homogenized milk.
Yet a further object of the present invention is to provide a treatment for the effective and economical reduction of defects attendant to UHT treated milk due to heat-resistant proteases.
These and other objects of the present invention will be apparent from the following discussion.